UA can always move a departure time because of a delay and many times when they had a delay time of lets say 2:00 PM they actually changed it and had the plane leave at 1 PM. There were a lot of angry pax who missed the flight expecting it to leave at 2.

It is more likely to have a replacement plane at a hub where they have planes sitting around or can tweak another departure time due to a light load.

But UA does say to wait in the gate area during a delay.